Grinders for grinding waste material such as trees, brush, stumps, pallets, railroad ties, peat moss, paper, wet organic materials and the like are well known. An example of one such prior art grinder, known as a tub grinder, is shown in commonly assigned U.S. Pat. No. 5,507,441 dated Apr. 16, 1996. Another example is shown in U.S. Pat. No. 5,419,502 dated May 30, 1995. Another type of grinder is known as a horizontal grinder, examples can be found disclosed in U.S. Pat. No. 5,975,443, U.S. Pat. No. 5,947,395, U.S. Pat. No. 6,299,082.
There are 4 different types of grinders that can be identified as defined in U.S. Pat. No. 6,299,082 including chippers, hammer mills, hogs and shredders: Each including a type of a rotary grinding device.
Tub grinders typically include a rotary grinding devices such as a hammermill or hog that is mounted on a frame for rotation about a horizontal axis. The hammermill or hog function in cooperation with a shear bar or anvil and typically a screen; the assembly including the hammermill or hog, anvil and screen forming a grinding device. A rotating tub surrounds the grinding device. The tub rotates about a generally vertical axis. Debris is deposited in the rotating tub and the grinding device grinds the debris.
FIG. 1 illustrates one type of prior art hammermill 20 commonly used with conventional tub grinders. The hammermill 20 includes a plurality of hammers 22 secured to a plurality of rotor plates 24. The rotor plates 24 are rotatably driven about a generally horizontal axis of rotation 26. Cutters 25 (e.g., cutter blocks, cutter teeth, etc.) are mounted on the hammers 22 (e.g., with nuts 30 and bolts 28). The hammers 22 are secured between the rotor plates 24 by shafts or rods 31 aligned generally parallel to the horizontal axis of rotation 26. For example, each hammer defines two holes 32 and 34 each positioned to receive a different shaft 31 (only one shown). Shims 36 are mounted between the hammers 22 and the rotor plates 24. When the rotor plates 24 are rotated about the axis of rotation 26, the hammers 22 are carried by the rotor plates 24 in a generally circular path. Material desired to be ground is fed into the circular path such that the material is impacted and reduced in size by the cutters 25 of the hammers 22. The grinding device of a conventional tub grinder also typically includes a sizing screen that curves along a lower half of the hammermill. FIG. 15 illustrates a grinding device typical of the prior art including a rotary grinder 20, anvil 100 and screen 102. In this particular embodiment the screen 102 is comprised of 2 portions to aid removal and replacement. They are made to be replaceable, as different screens are installed to achieve differing ground material sizes.
The screens 102 are supported in alignment with the rotary grinder by plates 104 that are located on the sides of opening 45 in the floor 44 corresponding to the ends of the rotary grinder 20, and in the vicinity of the rotary grinder support bearings. They are supported by frame 48. Anvil 100 is supported by the frame 48 and by the screen 102. The screens 102 are available in the prior art in a variety of configurations. One variety include round holes, another includes square or rectangular holes. The size of the holes varies, and effects the maximum size material that is allowed to pass through. Other variations of the screens include varying circumferential coverage wherein the length of screen is reduced, thereby increasing the gap 106 between the screens. It is known to significantly increase the gap 106 to allow material to exit the grinding device to reduce drag and power requirements. This is typically done in applications wherein the size of the ground material is not critical.
A grinding chamber is formed between the screen and the hammermill. The screen performs a sizing function and defines a plurality of openings having a predetermined size. In use, material desired to be ground is repeatedly impacted by the hammers 22 against the screen, or crushed between the hammers 22 and the screen, causing the material to be reduced in size. When the material is reduced to a size smaller than the predetermined size of the openings defined by the screen, the material moves radially through the screen. Upon passing through the screen, the reduced material commonly falls by gravity to a discharge system located beneath the hammermill 20.
The grinding device of a horizontal grinder typically includes an anvil and a screen. Many different configurations for horizontal grinders have been developed, but the basic grinding actions are similar to those found in tub grinders.
The typical prior art hammermills or hogs generally utilize block-shaped cutters mounted such that the effective cutting edge is parallel to the axis of rotation. This results in a surface of rotation for each cutter describing a cylinder, having a single effective cutting diameter that cooperates with the straight edge of the anvil.
Many other techniques have been developed to improve the cutting efficiency including U.S. Pat. No. 4,066,216 disclosing relatively narrow cutters with plates that project into the space between cutters and U.S. Pat. No. 3,580,517 disclosing sharp-pointed cutters with an anvil that matched the profile of the surface of rotation defined by the cutters. In both of these examples the cutters are not as robust as a standard block-type cutter, resulting in concerns related to durability. Hammer wear is a significant concern relating to hammermills. For example, hammer wear results in loss of hammer integrity, out-of-balance conditions, reductions in grinding efficiency, and increases in maintenance and service costs. With a conventional hammermill, it is difficult to replace the hammers because the hammermill must be disassembled. Disassembling a hammermill can be particularly labor intensive and time consuming because the rods used to connect the hammers to the hammermill are quite heavy. There are typically several rods per hammermill and frequently two rods must be removed to replace a single hammer. Furthermore, rods can be corroded in place or deformed thereby making it even more time consuming and costly to disassemble a hammermill.
Power requirements and resulting fuel consumption is also affected by the interaction of the screens and the hammers. The crushing characteristic is known to result in a significant amount of frictional drag. This drag results from to the tendency to trap the material between the stationary screen surface and the moving cutters or hammers while under significant load. This condition results in either the material moving with the cutters and sliding against the screen or the material being retained by the screen and the cutters sliding past the material or some combination. Any of these result in significant drag, thus grinders typically require significant power.